The present invention relates to a method and device for producing low temperatures, particularly with a closed-circuit refrigerating machine.
Refrigerating machines with small refrigerating power for reaching very low temperatures without an additional supply of refrigerant, i.e. autonomously, are being employed at an increasing rate in laboratories and industry, for example, as cryopumps in vacuum apparatus. Gaseous high-pressure helium is used as the coolant.
Such refrigerating machines mostly are based on the Stirling or Gifford-McMahon methods. These two methods differ from each other mainly in that in the first one, the work done on expansion and effecting the refrigeration is partly recovered. This is obtained by moving a displacer in synchronism with a compressor piston, which is effected by mechanical coupling. This results in a relatively high efficiency, but also in the drawback that no inexpensive, high-speed compressor can be employed, because of the limited, relatively low frequency of the cycle and the coupling of the displacer. The low speed requires a correspondingly large gyrating mass of the machine since this alone ensures a satisfactory energy accumulation during the expansion phase. That is why the manufacturing costs of such refrigerating machines are relatively high. In addition, standard component parts cannot be used for constructing refrigerating machines having different capacities and temperature ranges.
The Gifford-McMahon method bypasses this problem by uncoupling the compressor from the refrigerating machine. In this method, the high-pressure gas is supplied cyclically with the displacer motion during the compression phase, from a high-pressure accumulator through externally controlled valves and is then recycled during the expansion phase to a low-pressure accumulator. The sole task of the compressor thus is to compress the low-pressure gas and supply it again to the high-pressure accumulator. That is why the Gifford-McMahon method is a simple, inexpensive technical solution which also is reliable in operation. A disadvantage is only the higher energy consumption.